Methods of fabricating highly conductive regions in semiconductor substrates for radio frequency applications

ABSTRACT

Methods of fabricating highly conductive regions in semiconductor substrates for radio frequency applications are used to fabricate two structures: (1) a first structure includes porous Si (silicon) regions extending throughout the thickness of an Si substrate that allows for the subsequent formation of metallized posts and metallized moats in the porous regions; and (2) a second structure includes staggered deep V-grooves or trenches etched into an Si substrate, or some other semiconductor substrate, from the front and/or the back of the substrate, wherein these V-grooves and trenches are filled or coated with metal to form the metallized moats.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. §119(e) of thefollowing co-pending and commonly-assigned U.S. Provisional PatentApplication Ser. No. 60/331,854, entitled “METHODS OF FABRICATING HIGHLYCONDUCTIVE REGIONS IN SEMICONDUCTOR SUBSTRATES FOR RADIO FREQUENCYAPPLICATIONS,” filed on Nov. 20, 2002, by KingNing Tu, Ya-Hong Xie andChang-Ching Yeh, which application is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to fabricating semiconductor devices, and moreparticularly, to methods of fabricating highly conductive regions insemiconductor substrates for radio frequency applications.

2. Description of the Related Art

It has been a recent trend in the Si (silicon) integrated circuitindustry to integrate radio transmitters and other radio frequency (RF)devices onto digital integrated circuits. Such integration requires RFshielding to prevent interference with other noise sensitive portions ofthe integrated circuits.

Oxidized porous Si has been used to provide effective DC (directcurrent) isolation. However, oxidized porous Si cannot be made too thickbecause of the thermal expansion coefficient mismatch between oxidizedporous Si and Si. Therefore, it cannot be used for effective RFshielding, similar to silicon nitride and silicon dioxide films. On theother hand, the use of unoxidized porous Si as an insulating materialhas been successful in reducing RF crosstalk to a level identical tothat across a vacuum.

However, there is a need in the art to further reduce crosstalk forhigh-end RF applications. The present invention satisfies that need.

SUMMARY OF THE INVENTION

The present invention describes methods of fabricating highly conductiveregions in semiconductor substrates for radio frequency applications.These methods are used to fabricate two structures: (1) a firststructure includes porous Si regions extending throughout the thicknessof an Si substrate that allows for the subsequent formation ofmetallized posts and metallized moats in the porous regions; and (2) asecond structure includes staggered deep V-grooves or trenches etchedinto an Si substrate, or some other semiconductor substrate, from thefront and/or the back of the substrate, wherein these V-grooves andtrenches are filled or coated with metal to form the metallized moats.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 illustrates a structure for incorporating highly conductivemetallic regions into semiconductor substrates according to thepreferred embodiment of the present invention;

FIG. 2 illustrates an alternative embodiment of FIG. 1, wherein a highlyconductive metallized moat is formed by staggered deep V-grooves etchedinto the substrate;

FIG. 3 illustrates an alternative embodiment of FIG. 1, wherein a highlyconductive metallized moat is formed by a deep trench etched into thesubstrate;

FIGS. 4A, 4B and 4C are cross-sectional side views of the structures ofFIGS. 1, 2 and 3, respectively;

FIG. 5 is a flowchart illustrating the process steps used in creatingmetallized porous Si regions according to the preferred embodiment ofthe present invention;

FIG. 6 is a flowchart illustrating the process steps used in creatingV-grooves for a metallized moat according to the preferred embodiment ofthe present invention; and

FIG. 7 is a flowchart illustrating the process steps used in creatingtrenches for a metallized moat according to the preferred embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

FIG. 1 illustrates a structure for incorporating highly conductivemetallic regions into semiconductor substrates according to thepreferred embodiment of the present invention. A Si substrate 10 isdivided into a noisy circuit area 12 and a noise sensitive circuit area14, which are separated by a metallized moat 16. The highly conductivemetallized moat 16 is formed from metallized porous Si regions extendingthrough the thickness of the substrate 10. The noisy circuit area 12also includes metallized posts 18, created from metallized, localizedporous Si regions and extending through the thickness of the substrate10, wherein the metallized posts 18 act as “true ground” points.

FIG. 2 illustrates an alternative embodiment of FIG. 1, wherein thehighly conductive metallized moat 16 is formed by staggered deepV-grooves etched into the Si substrate 10, from both the front and theback of the substrate 10, wherein these V-grooves are filled with metal.

FIG. 3 illustrates an alternative embodiment of FIG. 1, wherein thehighly conductive metallized moat 16 is formed by a deep trench etchedinto one side (either front or back side) of the Si substrate 10,wherein the trench is also filled with metal.

FIGS. 4A, 4B and 4C are cross-sectional side views of the structures ofFIGS. 1, 2 and 3, respectively. FIG. 4A illustrates the metallizedporous Si regions 16 or 18 extending through the substrate 10, FIG. 4Billustrates the staggered deep V-grooves 16 etched into the Si substrate10 from both the front and the back of the substrate 10, and FIG. 4Cillustrates the deep trench 16 etched into one side of the Si substrate10.

Within the realm of integrated circuit technology, there are twopotential applications using the present invention. Both applicationsaddress important issues associated with mixed-signal integratedcircuits, which comprise a family of newly emerged type of integratedcircuits that is used for cellular telephones, portable electronics,high speed modems, and data storage devices, such as computer harddrives.

A first application provides one or more metallized posts 18 on thesubstrate as low impedance paths to ground which is typically located atthe backside of the chip. These posts 18 can be used as “true ground”points, i.e. points with very low impedance contact to the groundpotential outside the chip, across the substrate 10. Such posts 18 havevery short paths to ground points, as compared to typical ground linesin conventional Si integrated circuit technology. As a result, theseposts 18 have much lower impedance to ground, especially for highfrequency signals.

A second application creates the metallized moat 16 from a metallizedporous Si region. Alternatively, the second application creates themetallized moat 16 by etching deep V-grooves or trenches in the Sisubstrate 10 and then depositing metals in the V-grooves orelectroplating the trenches. In this second application, the metallizedmoat 16 shields the noise sensitive circuits 14 from high frequencynoise generated by the noisy circuits 12. The metallized moat 16, inessence, creates a conducting cage, which is an electromagnetic shieldthat reduces RF crosstalk between the circuits 12 and 14.

As noted above, metallized porous Si regions can be used for both thefirst and second applications, whereas the metal-filled deep V-groovesand trenches are more suited for the noise isolation of the secondapplication. Moreover, while the preferred embodiment uses Sisubstrates, other semiconductor substrates, such as GaAs (galliumarsenide) and InP (indium phosphide), may be used in alternativeembodiments, especially in applications involving the V-grooves andtrenches.

FIG. 5 is a flowchart illustrating the process steps used in creatingthe metallized porous Si regions according to the preferred embodimentof the present invention.

Block 20 represents the formation of porous Si regions being performedby anodization, which is a well-known art that was first invented abouthalf a century ago. In this step, the surface of the Si substrate isexposed to HF (hydrogen fluoride) containing an electrolyte. Porous Siforms into the Si substrate when an electrical current is passed throughthe Si-electrolyte interface with the Si substrate acting as the anode.Adjusting the HF concentration in the electrolyte and the currentdensity during anodization alters the microstructure of the porous Siregion so formed.

Block 22 represents the metallization of the porous Si regions beingperformed. The porous Si regions, with their interconnected pores,provide an excellent skeleton for metal deposition. Metals can beintroduced into the porous Si regions in a number of different ways: byvapor deposition, solid state interdiffusion and reaction, and liquidstate penetration. Because the porous Si regions each have a very largeinterconnected internal surface area, a capillary effect can be used tofacilitate the penetration of any low melting point molten metal thatwets the surface of the Si substrate throughout the entire porous Siregion.

Two metals that fit the low melting point requirement are Au (gold) andAl (aluminum). The eutectic point of Au—Si is 370 degrees C. and that ofAl—Si is 577 degrees C. Both metals are acceptable with respect toprocessing of integrated circuit devices on Si substrates.

Since Au is a deep trap impurity in Si, the Au penetration is followedwith a penetration by molten Sn (tin) or an Sn-based solder. Sn willfill up the pores of the porous Si regions. Moreover, Sn serves theimportant function of retaining Au from outdiffusion, because of Au—Snintermetallic compound formation. Sn also provides mechanical strengthto the otherwise porous structure.

FIG. 6 is a flowchart illustrating the process steps used in creatingthe V-grooves for the metallized moat 16 according to the preferredembodiment of the present invention.

Block 24 represents the V-grooves being created along a [110] directionon an (001) surface of the Si substrate can be created using standardlithography techniques, followed by an anisotropic wet etching insolutions such as KOH (potassium hydroxide). The step may prepareV-grooves on both surfaces of the Si substrate for isolation purposes.The width of the V-groove is selected to give a depth of the V-groovethat is about half the thickness of the substrate.

Block 26 represents the metallization of the V-grooves being performed.A lift-off process is used to deposit a multilayer metallic thin film(the total thickness of which is preferably on the order of a fewhundred nanometers) into the V-grooves. For example, a trilayer ofCr/Cu/Au (chromium/copper/gold) or Ti/Ni/Pd (titanium/nickel/palladium)can be used. The resulting structure can be strengthened by a flow ofmolten solder into the V-grooves using a horizontal capillary effect.The solder may be Pb-free (lead-free) alloys such eutectic SnAg(tin-silver) or SnAgCu (tin-silver-copper) with a melting point around220 degrees C.

FIG. 7 is a flowchart illustrating the process steps used in creatingthe trenches for the metallized moat 16 according to the preferredembodiment of the present invention.

Block 28 represents the trenches being created along a [110] directionon an (001) surface of the Si substrate can be created using standardlithography techniques, followed by an anisotropic wet etching insolutions such as KOH.

Block 30 represents the metallization of the trenches being performed.Preferably, the Cu is deposited into the trenches by electro-plating orchemical vapor deposition (CVD). The entire trench can be filled withCu, or a layer of Cu can be deposited and the rest of the trench filledwith molten solder.

Conclusion

This concludes the description of the preferred embodiment of theinvention. The foregoing description of one or more embodiments of theinvention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching. It is intendedthat the scope of the invention be limited not by this detaileddescription, but rather by the claims appended hereto.

1. A method of fabricating highly conductive regions in semiconductor substrates for radio frequency applications, comprising: forming one or more porous Si (silicon) regions on a Si substrate by anodization; and isolating a noisy circuit area on the Si substrate from a noise sensitive circuit area on the Si substrate by depositing one or more metals into the porous Si regions to create a metallized, localized porous Si region comprising a moat or trench that extends through the Si substrate and laterally separates the noisy circuit area from the noise sensitive circuit area on the surface of the Si substrate.
 2. The method of claim 1, wherein the forming step comprises: exposing a surface of the Si substrate to HF (hydrogen fluoride) containing an electrolyte; and passing an electrical current through a Si-electrolyte interface with the Si substrate acting as the anode in order to form the porous Si regions on the Si substrate.
 3. The method of claim 2, further comprising adjusting the HF concentration in the electrolyte and the electrical current density during anodization to alter a microstructure of the porous Si regions.
 4. The method of claim 1, wherein the metals are deposited into the porous Si regions by vapor deposition, solid state interdiffusion and reaction, or liquid state penetration.
 5. The method of claim 1, wherein the metal is Au (gold) or Al (aluminum).
 6. The method of claim 5, wherein a penetration by Au is followed with a penetration by molten Sn (tin) or an Sn-based solder.
 7. The method of claim 1, further comprising creating one or more metallized posts, in the noisy circuit area, from the metallized, localized porous Si regions, wherein the metallized posts extend through the substrate and act as true ground points. 